Group III nitrides, in particular, GaN, are of interest for many electronic and optoelectronic applications, including high temperature electronic devices, high-power microwave circuits, blue lasers and LEDs, as well as solar-blind photodetectors. For these applications, the semiconductor layers are grown epitaxially on a crystalline substrate. After the epitaxial growth, a masking layer is typically deposited on the surface and lithographically patterned. The pattern can then be transferred to the underlying semiconductor material by etching. There are several reasons why this patterning is needed, including device isolation, contact to underlying layers, and the formation of waveguides and laser facets.
GaN etching is commonly performed using a reactive ion etching (RIE) technique. An article entitled "Dry and Wet Etching for Group III-nitrides" by I. Adesida et al. appearing in MRS Internet Journal of Nitride Semiconductor Research, Vol. 4S1, No. G1.4, 1999, discussed in detail various prior art RIE and wet chemical etching techniques for GaN. One of the drawbacks of reactive ion etching includes the generation of ion-induced damage on the surface of the GaN film. Wet chemical etching produces significantly less damage and is often less costly and complex than RIE systems, thus making wet chemical etching an attractive alternative. However, epitaxial GaN (or Group III-nitrides in general) has been found to be resistant to wet chemical etching. In particular, the etch rates have been too slow for efficient processing. Further, the wet etch techniques have generally not been directional enough to produce sidewalls that mirror the masking material. To circumvent this problem, photo-enhanced electrochemical PPEC) has been developed. U.S. Pat. No. 5,773,369 issued to E. L. Hu et al on Jun. 30, 1998 discussed an exemplary PEC process for Group III nitrides where an ultra-violet light source and a metal mask layer is used. A bias is applied to the metal mask to effect the etch process. In general, a PEC process is ineffective in etching p-type and semi-insulating materials since there are not enough holes at the surface to allow the wet chemistry to progress and etch the material. Regardless of the process used, the etch depth is controlled by several factors, including--among others--the etch time, etch composition and sample temperature.
In some applications, it is desirable to perform an additional growth over already patterned semiconductor layers. With existing etch processes, it is very difficult to pattern any new layer selectively without damaging the previously grown layers. A need remains in the prior art, therefore, for a more controllable method of selectively regrowing and patterning Group III-nitride layers. Selective regrowth would, for example, enable the integration of various GaN-based devices on a single wafer (e.g., electronic devices with lasers or LEDs).